Minerals and Rocks

Lecture Outline

What are minerals?

Common rock-forming minerals

Physical properties of minerals

Basic rock types

The rock cycle

Minerals

Natural Solid Atoms arranged in orderly repeating 3D

array: crystalline Not part of the tissue of an organism Composition fixed or varies within defined

limits

Minerals are the “building blocks” of rock

A mineral is a naturally occurring, solidcrystalline substance, generally inorganic,with a specific chemical composition

Large individual crystals (rare)

Mass of small grains: each is a crystal, but grown up against each other

Atomic Structure of Minerals NaCl - sodium

chlorideHalite

Chemical Bonds: Ionic

Electrical attraction between ions of opposite charge Bond strength increases with the electrical charges of the

ions Bond strength decreases as the distance between the

ions increases Most minerals are this kind of compound

Na+ Cl-

Ionic Bonding example:halite

AnionCation

Covalent Bonds: Electron sharing Generally stronger than ionic bonds (e.g., diamond)

Crystallization of Minerals

Need starting material with atoms that can come together in the proper proportions Growth from a liquid or a gas

Time and space for crystallization

Appropriate temperature and pressure

Examples Magma that has cooled below its melting point Supersaturated solution --> precipitation

Crystallization of Minerals

Crystals begin as an initial “seed” - a microscopic crystal

Atoms keep being added in a 3D array, repeating the basic arrangement

Crystal faces are based on the array structure

Cations and Anions

Anions are typically large

Cations are relatively small

Crystal structure is determined largely by the arrangement of the anions

Common cations and anions

Radii given in angstroms; 10-8 cm

Ions can be compound So far, we’ve talked about individual atomic

ions Many common minerals are silicates

SiO44-

Complex ions act as a single ion in forming crystal structure

Cation Substitution

Crystal structure determined by those large anions

Various cations can substitute for each other in many minerals Same crystal structure Different chemical composition

PolymorphsMinerals with the same composition,

but different crystal structure.

Common Rock-Forming MineralsMinerals fall into a small number of related “families” based mainly on the anion in them

Silicates Most abundant minerals in the Earth's crust Silicate ion (tetrahedron), SiO4

4-

Quartz (SiO2), K-feldspar (KAlSi3O8), olivine ((Mg, Fe)2SiO4), kaolinite (Al2Si2O5(OH)4)

Quartz (SiO2)

Silicate structure Most of the most common rocks in the crust

are silicates Silicate tetrahedra can combine in several

ways to form many common minerals Typical cations:

K+, Ca+, Na+, Mg2+, Al3+, Fe2+

Different numbers of oxygen ions are shared among tetrahedra

Carbonates

Cations with carbonate ion (CO32-)

Calcite (CaCO3), dolomite (CaMg(CO3)2), siderite (FeCO3), smithsonite (ZnCO3)

Make up many common rocks including limestone and marble

Very important for CCS!

Calcite (CaCO3)

CaCO3 + 2H+ = Ca2+ + CO2 + H2O

Smithsonite (ZnCO3)

Oxides

Compounds of metallic cations and oxygen

Important for many metal ores needed to make things (e.g., iron, chromium, titanium)

Ores are economically useful (i.e., possible to mine) mineral deposits

Hematite (Fe2O3)

Sulfides

Metallic cations with sulfide (S2-) ion Important for ores of copper, zinc, nickel, lead, iron Pyrite (FeS2), galena (PbS)

Galena (PbS)

Sulfates

Minerals with sulfate ion (SO42-)

Gypsum (CaSO4.H2O), anhydrite (CaSO4)

Gypsum

Cave of the Crystals

• 1,000 feet depth in the silver and lead Naica Mine

• 150 degrees, with 100 % humidity

• 4-ft diameter columns 50 ft length

Gypsum

Identification of Minerals

Chemical composition (microprobes and wet chemical methods)

Crystal structure (X-ray diffraction)

Physical properties

Physical properties

Hardness

Physical properties Hardness

Cleavage: tendency of minerals to break along flat planar surfaces into geometries that are determined by their crystal structure

Cleavage in mica

Cleavage in calcite

Halite (NaCl)

Physical properties

Hardness

Cleavage

Fracture: tendency to break along other surfaces (not cleavage planes)

Conchoidal fractures

Physical properties Hardness Cleavage Fracture Luster (metallic, vitreous, resinous, earthy, etc.) Color (often a poor indicator; streak color is better) Specific gravity Crystal habit (shape)

Rocks

An aggregate of one or more minerals; or a body of undifferentiated mineral matter (e.g., obsidian); or of solid organic matter (e.g., coal)

More than one crystal Volcanic glass Solidified organic matter Appearance controlled by composition and size and

arrangement of aggregate grains (texture)

Rock Types

Igneous Form by solidification of molten rock (magma)

Sedimentary Form by lithification of sediment (sand, silt, clay,

shells) Metamorphic

Form by transformations of preexisting rocks (in the solid state)

Igneous Rocks

Intrusive Extrusive

Intrusive (plutonic)

Form within the Earth Slow cooling Interlocking large crystals Example = granite

Extrusive (volcanic)

Form on the surface of the Earth as a result of volcanic eruption

Rapid cooling Glassy and/or fine-grained texture Example = basalt

Basalt: igneous extrusive

Intrusive and extrusive igneous rocks

Sedimentary Rocks

Origin of sediment

Produced by weathering and erosion or by precipitation from solution

Weathering = chemical and mechanical breakdown of rocks

Erosion = processes that get the weathered material moving

Sediment types

Clastic sediments are derived from the physical deposition of particles produced by weathering and erosion of preexisting rock.

Chemical and biochemical sediments are precipitated from solution.

Clastic

Chemical/biochemical

Lithification

The process that converts sediments into solid rock Compaction Cementation

Cemented sandstone

Metamorphic Rocks

Regional and contact metamorphism

conglomerate

metaconglomerate

granite

gneiss

The Rock Cycle

The Rock Cycle